A luminaire or lighting fixture is a mechanical assembly of parts which must be capable of disassembly for lamp replacement, and in practice it is impossible to have it perfectly hermetically sealed. Therefore, it is accepted that even with hazardous location luminaires, any ambient gas will in time find its way into the luminaire. Lighting fixtures listed by Underwriters Laboratories (UL) as suitable for use in hazardous locations in which combustible gases or vapors are present (Class 1, Division 1), are required to have enclosures for the electrical components having sufficient strength to withstand the explosion pressure should there be an electrical or other malfunction that ignites the gases inside. Furthermore, as the momentary pressure from the explosion inside relieves itself to the outside, the ignited gases must be cooled sufficiently as they exit that explosive gases on the outside are not ignited, such quality being commonly referred to as nonflame propagation through the joints.
The lamp or light source proper within the luminaire may be either a filament or an inner arc tube in the case of a high intensity discharge (HID) lamp. In either case the lamp envelope operates at a very high temperature and usually is merely thin (1 mm) and relatively fragile glass. Accordingly, a globe is needed to surround and shield the lamp envelope from damage while permitting the light to pass out. The globe also serves to keep ignitable gases or other combustible material away from the high temperature surface of the lamp envelope itself.
The globe is usually made of pressed glass at least 1/4 inch thick and in order for the globe to safely accomplish its functions, the glass needs to be tempered. Tempering as applied to glass is a process that entails first heating the material uniformly to a temperature adequate to relax internal stresses but permitting handling without deformation. The glass is then rapidly quenched by extracting heat from both surfaces at rates generating a symmetrical temperature profile across its thickness. This is continued until the hottest point on the profile is below the effective solidification temperature. Finally the glass is cooled to room temperature, and in such final cooling, the contraction of the relatively hot core is resisted by the cool surface layers. The result is a generally parabolic stress distribution across the thickness of the glass; compression near the surface is balanced by tension in the interior, and the maximum tensile stress is about half the compressive stress at the surface.
Glass fractures when its tensile strength is exceeded and it is the outer surface that is vulnerable. The term "outer" is used with reference to the glass itself and in a bowl, both inside and outside are "outer" surfaces. With tempered glass, an external force such as a blow that would put an outer surface in tension must first neutralize the compressive prestress before any net tensile stress can develop at the surface, and thus breaking strength is enhanced. When tempered glass is broken, the strain energy reduces the glass into harmless small fragments more or less cubic in shape.
In a globe for a hazardous area luminaire, the desideratum is to have the breaking strength as high as possible. However, increasing the wall thickness or the degree of tempering does not necessarily improve the breaking strength and may, in fact, diminish it. The reason is that the strength of glass ultimately is limited by the maximum stress of approximately 10,000 p.s.i. that it can carry in tension. Excessive tempering produces a high tensile stress condition hidden as a weakness inside the glass and making it effectively less resistant to an external force. The presence of a stone, that is a non-homogeneous fragment in the glass having a different coefficient of expansion, may aggravate the situation. Globes having such inherent weaknesses occasionally spontaneously shatter or self-destruct, and thus present a real hazard with the possibility of disaster should there be explosive gas present in the environment at the time.